There are known techniques for obtaining a desired device structure in a semiconductor fabrication process as follows: a plasma is produced in a processing chamber in which a silicon wafer (hereinafter referred to as a “wafer”) as a substrate to be processed is placed, and plasma processing such as deposition, ion implantation, and etching is carried out on the substrate using the plasma. Such a plasma processing system is provided with a substrate holding device including a so-called electrostatic chuck in order to align and hold a substrate in the processing chamber.
As an electrostatic chuck, a so-called bipolar electrostatic chuck in which a dielectric chuck plate is attached to an upper surface of a chuck main body having positive and negative electrodes embedded therein has been disclosed in, for example, Patent Document 1. Moreover, a substrate held by the substrate holding device is controlled to be at a predetermined temperature in some cases depending on the plasma processing to be carried out in the processing chamber. In such a case, the following chuck has been known: for example, electrical resistance heating means is incorporated in a chuck main body (or pedestal), and a chuck plate is configured such that a rib portion is formed which comes in surface contact with a peripheral edge portion of a back surface (on the opposite side from a surface on which predetermined processing is performed) of a wafer, and that multiple support portions are provided upright and arranged, for example, concentrically in an internal space surrounded by the rib portion.
In this substrate holding device, when the wafer is heated or cooled, an assist gas such as an Ar gas is supplied to the internal space through a gas passage formed in the chuck main body, and a gas atmosphere is formed in the internal space defined by the rib portion and the back surface of the wafer. Thus, heat transfer from the chuck main body to the wafer is assisted, and the wafer can be efficiently heated or cooled. Moreover, depending on the plasma processing, a configuration may be employed in which a bias voltage can be applied to a substrate clamped by a substrate holding device.
In recent years, there has been a trend toward using wafers having large diameters and small thicknesses (700 nm or less) in order to further improve productivity. Such a wafer has warpage, and the warpage state of the wafer is changed by the heating or cooling of the wafer or by stress in a film deposited on a surface of the substrate by plasma processing. In some cases, the warpage state of the wafer is changed by stress in the film even during plasma processing.
If voltages applied to the positive and negative electrodes are fixed in the process of clamping such a warping wafer or during plasma processing, there may arise problems: for example, an alignment failure occurs due to a too weak clamping force in the process of clamping the wafer, and damage is caused due to a too strong clamping force. Also, in the case of a warping wafer, the distance from the upper surface of the chuck main body to the back surface of the substrate varies across the surface of the substrate according to the warpage state of the substrate. Accordingly, the amount of heat transfer through the inert gas atmosphere varies in the process of heating or cooling. As a result, the substrate temperature is non-uniform across the surface of the substrate. When predetermined plasma processing is performed in a state in which the substrate temperature is non-uniform as described above, there is also a problem that the thickness and quality of a film vary.
Accordingly, the inventors of the present invention have proposed that the substrate holding device includes an AC power supply for passing an alternating current through the capacitance of a chuck plate, first measuring means for measuring a current value of the alternating current passing through the capacitance of the chuck plate, second measuring means for measuring a gas flow rate of the gas when the gas is caused to flow by gas introduction means, and control means for controlling a DC voltage applied between the two electrodes so that at least one of the current value and the gas flow rate measured with the first and second measuring means may be in a predetermined range (see Japanese Patent Application No. 2008-297295).
According to this, after a substrate is clamped by the chuck plate, the warpage state of the substrate can always be correctly grasped from the gas flow rate and/or an impedance (alternating current value) even if the substrate has warpage in either direction. Further, the substrate can be clamped with an appropriate clamping force according to the warpage state of the substrate by storing in advance in the control means the gas flow rate range and/or the alternating current value range under the condition in which a substrate is clamped in a warpage-free state, and, based on the ranges, controlling the DC voltage (i.e., the substrate clamping force) applied between the two electrodes so that at least one of the current value and the gas flow rate measured with the first and second measuring means may be in a predetermined range.
However, when the substrate holding device configured as described above is used in a plasma processing system, there are cases where the state of the substrate cannot be monitored based on the impedance. Also, it has been revealed that there is the following problem: even if the DC voltage applied between the two electrodes is controlled based on the amount of change in the impedance, a too strong substrate clamping force may damages this substrate, or a too weak substrate clamping force forms a local gap between the upper surface of the chuck main body and a clamped surface of the substrate, thus inducing an abnormal discharge.